Several reports have been published describing the presence of volatile N-nitrosamines in various rubber products. The present concern about the occurrence of volatile N-nitrosamines in baby bottle rubber nipples and the possible migration of these compounds into infant formula was prompted by a report of Preussmann et al., (1981) Am. Chem. Soc. Symp. Ser. 174, American Chemical Society, Washington, DC, p. 217.
A method was described for the estimation of volatile N-nitrosamines in the rubber nipples of babies bottles. In study of rubber nipples from one manufacturer, N-nitrosopiperidine were determined by gas chromatography, using a thermal energy analyser, and their presence was confirmed by mass spectrometry with average levels of individual nitrosamines ranging from 22 to 281 ppb. When the nipples were sterilized in a conventional sterilizer together with milk or infant formula the three nitrosamines migrated into the milk or formula. Storing a bottle of milk with a rubber nipple invertied in it for 2 hr at room temperature or overnight in a refrigerator after sterilization resulted in an 8-13% average increase in the nitrosamine levels migrating into the milk. On repeated sterilization of a single nipple, the quantities of nitrosamines migrating into milk from rubber nipples declined steadily, but after seven sterilizations, nitrosamines were still readily detectable in the milk. Nitrosamine levels were higher in rubber nipples after sterilization, indicating the presence of nitrosamine precursors in the nipples. No nitrosamines were found in raw, uncured rubber. Chemical accelerators and stabilizers added during the vulcanization process are the source of the amine precursors in rubber nipples.
On Jan. 1, 1984, the U.S. Food and Drug Administration (hereinafter "FDA") established an action level of 60 ppb total N-nitrosamines in rubber nipples. The action level was reduced to 10 ppb on Jan. 1, 1985.
A collaborative study was conducted on the FDA dichloromethane extraction method for determining volatile N-nitrosamines in baby bottle rubber nipples. Following dichloromethane extraction, N-nitrosamines were determined by gas chromatography-thermal energy analysis. Six pairs of blind duplicate rubber nipple samples representing 6 lots were analyzed by 11 collaborating laboratories. All samples were portions taken from equilibrated composites of cut-up rubber nipples obtained from manufacturers in the United States. Recoveries of the internal standard (N-nitrosodipropylamine) at approximately 20 ppb ranged from 10 to 120%. Reproducibility relative standard deviations (RSD) were between 35 and 45% for N-nitrosamine levels from 10 to 20 ppb. However, when data from laboratories with recoveries less than 75% were excluded (this is now specified in the method), RSD, values were between 11 and 32% for N-nitrosamine levels from 6 to 26 ppb. Values were consistent with or better than those reported for other analytical techniques designed to quantitate trace contaminants at the low ppb level, e.g., aflatoxin in food. The method has been adopted official first action for the quantitation of volatile N-nitrosamines in baby bottle rubber nipples. See Gas Chromatographic-Thermal Energy Analysis Method for Determination of Volatile N-Nitrosamines in Baby Bottle Rubber Nipples: Collaborative Study, by Gray & Stachiw, J. Assoc. Off. Anal. Chem. (1987) 70, March Issue.
Although research in the rubber industry has been devoted to lowering or eliminating nitrosamines, none of these studies have included silicone rubber materials. Silicone elastomeric compositions, in contrast to synthetic rubber compounds, are usually prepared from a vinyl-containing polydiorganosiloxane, an organohydrogensilicone crosslinker, and a platinum catalyst. The compositions of this type are desirable for many reasons. For instance, they cure without by-products. They can cure at room temperature or at elevated temperatures. They can be stabilized for storage at room temperature by utilization of a platinum catalyst inhibitor. And, they can be made from high and low viscosity polymers. These compositions utilize components that are low enough in viscosity that they are easily pumpable or extrudable as well as have a fast cure time. These compositions also provide cured silicone elastomers which are characterized by high strength and high durometer.
Cross-linked silicone polymers with their particularly low intermolecular interactions have low tensile strengths. Only with the addition of reinforcing fillers can high-strength silicone polymers be obtained. Particularly suitable are fumed silicas with BET surface areas of 150 to 400 m.sup.2 /g which increase the tensile strength about 20 fold to 10-12 MPa. At the same time, viscosity is considerably increased because fumed silicas have a strong thickening effect. This effect is caused by formation of agglomerates of the primary silica particles. These agglomerates build a three-dimensional network (tertiary structure) via hydrogen bonds so that the bulk density of the fumed silica is only about 50 g/l. To produce a mixture of 40 parts silica, in 100 parts polymer therefore requires addition of 8 volume parts of filler for 1 volume part of polymer. These ratios dearly indicate the necessity of using treating agents which reduce interactions between filler agglomerates as much as possible. The most effective and most commonly used treating agents is hexamethyldisilazane (hereinafter "HMDZ"). The fillers treated with HMDZ have a considerably reduced thickening effect and therefore are particularly suitable for the use in liquid silicone rubbers.
Since silicone elastomer are entirely different polymers, these silicones became leading candidates to replace the synthetic rubber compounds, Analysis of the cured silicone elastomers showed no presence of nitrosamines. However, to applicants' surprise, upon post-baking as required by FDA, the presence of nitrosamines was detected. For silicones to serve these FDA regulated markets, a method of removing the nitrosamines must be found.